The input stage of an operational amplifier is, conventionally, a difference amplifier. The operational amplifier is a building block for instrumentation, analog control, and voltage regulation applications, to name a few. In a wide variety of applications, difference amplifiers are powered between voltage sources of opposite polarity. In operation, two input signals are connected to a difference amplifier for controlling the current flow through one of a symmetric pair of transistors. The pair of transistors are connected to a common current source, conventionally powered from the negative power supply. If the two input signals are within the allowable input voltage range, then a signal taken at the collector of one of the pair of transistors forms the output signal.
The voltage of the output signal corresponds to the difference in voltage of the input signals. When the input signals are not within the input voltage range of the difference amplifier, the output signal no longer corresponds to the difference in input voltages. When the difference amplifier is powered between supplies of equal and opposite polarity, the input signal range extends from a negative voltage through ground to a positive voltage.
Digital integrated circuits including logic circuits, microprocessors, and memory circuits, are conventionally powered between a single voltage source and ground. When a difference amplifier is operated between a single voltage source and ground, the input signal range does not include voltages near ground potential, but extends around a voltage of half the power supply voltage.
In many applications where a difference amplifier would be useful, supply voltages of opposite polarity are arranged as a prerequisite so that input signals near ground can be connected to the difference amplifier inputs. Such an arrangement is difficult in digital integrated circuits. A negative power supply voltage that is stable, invariant to temperature, and capable of supplying necessary current is difficult to design, occupies space on an otherwise digital integrated circuit substrate, and usually requires decoupling techniques to counteract the effects of noise generated by the digital circuitry.
Thus, there remains a need for a difference amplifier circuit operable between a single power supply and ground that can provide a difference voltage output signal when input signals have voltages near ground. The need is especially apparent in digital integrated circuit applications where use of a negative power supply is costly and temperature compensation of the difference amplifier output signal is important.